As you can see the vertices are non planar as they are unevenly distributed in the z axis. Is there anyway to make it into a circle like the edgeloop at the bottom of the mesh?
Yes, for straightening that circle you can do like @Duarte Farrajota Ramos said (selecting that circle and pressing S+Z+0)
But that topology will sooner or later cause issues, so I did a quick gif to show you an example of fixing that topology, it stretched a few lines but nothing too noticeable on the mesh.
What I did:
- I straightened that circle with S+Z+0)
- Since this object can be perfectly mirrored on all sides I deleted the left and back part to modify it faster.
- Then I started to merge some vertices using W>
- I duplicated that part and inverted with S(lock on an Axis)>
- And then I joined all parts using the Snapping Tool to Merge at the Closest Vertex.
- Finaly I Removed Doubles with W>